Hi, I now have some questions about single-screen mirroring. It seems like a pretty simple idea, but as I am implementing mapper #7 (all the rare games), I am convinced that I do not know what is going on.

Mapper #7 provides a mechanism for choosing where to mirror pages for one-screen mirroring, either $2000 or $2400. Can someone explain to me why this is releveant for one screne mirroring? Wouldn't both options essentially be the same, because a write to any NT would mirror to $2000 (or $2400), and the same would happen with a read? I do not understand how scrolling can work with games like this, apart from manually writing the scrolling graphics each VBLank, and I doubt that is how they work. Can somone help? Thanks a lot

I think that each game has a default mirroring in the header (vertical or horizontal). All writes to the nametable address space follow this mirroring criteria. However, when drawing the screen (i.e. "reading"), you only read from the the active "one-screen" specified by the mapper.

So if a mapper 7 game is loaded, you honor the mirroring it comes with by default (for all writing). This will always yeild different values in the nametables. If mapper 7 sets mirroring to 0x2000, then no matter what nametable you are drawing from (0x2000, 0x2400, 0x2800, or 0x2C00), you get your data from 0x2000.

I may be completely wrong on this, cuz I havn't yet gotten it working in my emulator.....

Have you gotten mapper 4 working? I have a few questions on it, if you do...

No, I haven't gotten mapper 4 yet, in fact I havent even started on it. I only have #2 and 3 done so far, with #7 being worked on now with #1 to follow once I get #7 working as it should. Thanks for the help

Almost, but not quite.
When using 1-screen mirroring, reads AND writes are affected. The mirroring bit in the header is completely ignored.

The significance is that you can dynamically select which nametable to render from simply by writing to the mapper (generally used in vertical-scrollable games for the status bar).

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Quietust
P.S. If you don't get this note, let me know and I'll write you another.

Thanks for the help. That is how I interpreted single-screen mirroring to work.

If anyone cares, I fixed my mapper #7 problems by having the PPU memory write to the first physical NT RAM area if the single screen mirroring is set to $2000, and to the second physical NT RAM area if the ss mirroring is set to $2400.

Big Pasty